The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to a method and system for processing nuclear magnetic resonance (NMR) signals acquired during a scan in order to remove transient spike noise from the NMR signals and thereby eliminate artifacts produced by such noise in the reconstructed image.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to an additional magnetic field (excitation field B.) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A radio-frequency (RF) signal, which is also denoted the nuclear magnetic resonance (NMR) signal, is emitted by the excited spins after the excitation signal B1 is terminated, and this NMR signal may be received and processed to form an image.
MRI scanners include a large magnet assembly for producing the uniform polarizing field B0 in a bore which is large enough to receive a patient. An RF coil surrounds the patient and is switched between a transmitter and receiver to produce the excitation field B, and to receive the resulting NMR signal. Additionally, three sets of gradient coils surround the RF coil to produce magnetic field gradients Gx, Gy and Gz, and a shield is disposed therebetween to isolate the RF coil so that its uniform field is not disrupted. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which the magnetic field gradients are switched on and off according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The NMR signals are very small and extraordinary measures are taken to shield the MRI system from external RF noise and to eliminate sources of internal noise. Nevertheless, generators of short-duration noise pulses persist and may elude location and elimination. These noise pulses are referred to as xe2x80x9cspike noisexe2x80x9d, xe2x80x9cimpulse noisexe2x80x9d or xe2x80x9cwhite pixelsxe2x80x9d, and lead to image artifacts with such vernacular names as corduroy and zebra artifacts. Sources of such noise include arcing due to partial discharges from intermittent electrical contacts or electrostatic discharge, and harmonics of fast transients such as those caused by ground loops. When such noise sources occur regularly, their source can be located and measures can be taken to eliminate them. This xe2x80x9chardeningxe2x80x9d process occurs at any new MRI installation, and eventually all the short-duration noise sources are eliminated except those which are intermittent and defy cost-effective diagnosis.
A number of strategies have been employed to mitigate the effects of intermittent noise sources. Such methods include the examination of the acquired NMR signals to locate noise spikes or the examination of the reconstructed image to locate the effects of such noise. These prior methods work when the noise spike occurs in NMR signals that are heavily phase or frequency encoded (i.e. on the edges of k-space), but they do not perform well when the noise spike occurs in NMR signals near the center of k-space. In the latter case the NMR signal magnitude is quite large and it is more difficult to discern signal from noise. Noise spikes detected by such methods are sometimes removed by interpolating between the adjacent values.
Another strategy, which is more effective in detecting and eliminating short-duration noise spikes near the center of k-space, was disclosed in U.S. Pat. No. 5,525,906 entitled xe2x80x9cDetection and Elimination of Wide Bandwidth Noise in MRI Signals.xe2x80x9d In that strategy, the NMR signal is processed by a noise filter or Transient Noise Suppression (TNS) system that includes a noise detector. The noise detector has a bandstop filter that is tuned to stop the NMR signals but to pass a range of frequencies outside the NMR imaging frequency band that include at least some of the spike noise. Because a considerable portion of the energy of short-duration spikes is located outside of the NMR imaging frequency band, the bandstop filter effectively isolates the spike noise from the NMR imaging frequency information. The bandstop filter thus provides an output signal that is an indication of the level of spike noise independent of the NMR imaging frequency information.
The magnitude of the output signal from the bandstop filter is then compared with a noise reference level at a comparator. When the magnitude of the output signal exceeds the noise reference level, a noise indication signal is produced (or is changed in its level) indicating that there is noise due to short-duration spikes. The noise indication signal can then be used to eliminate noise from the entire NMR signal by blanking out portions of the NMR signal whenever noise is detected, before the NMR signal is provided to an image reconstructor.
Although TNS systems are more effective at eliminating noise due to short-duration spikes near the center of k-space than the other systems mentioned above, TNS systems are highly frequency dependent. In particular, the stop band of the bandstop filter in a TNS system must be carefully set so that the filter passes the ranges of frequencies above and below the NMR imaging frequency band and not the NMR imaging frequency band itself. If the pass band of the bandstop filter encompasses the NMR imaging frequency band, the TNS system may mistake the high-magnitude signal components containing the imaging information for high-magnitude noise spikes, and inappropriately blank out portions of the NMR signal that contain useful information rather than noise. The high sensitivity of TNS systems to frequency is undesirable insofar as TNS systems must as a result be carefully and accurately implemented in order for the systems to properly remove noise due to short-duration spikes.
The high frequency sensitivity of TNS systems is also undesirable because it makes it necessary to configure a TNS system differently depending upon the frequency of operation of the MRI system (particularly the frequency of the polarizing field B0) in which it is implemented. Given the wide variety of MRI systems, and given that some MRI systems can operate at a variety of different frequencies, TNS systems must be repeatedly configured. Given that the tuning of TNS systems to MRI systems, and performance verification, can be costly, the high frequency sensitivity of TNS systems increases the cost of systems overall and places an undesirable constraint on the design of new MRI systems, particularly those that operate at multiple frequencies.
It would therefore be advantageous if a system could be developed for eliminating noise due to short-duration spikes from NMR signals and thereby mitigating the appearance of undesirable image artifacts from images created by MRI systems. It would particularly be advantageous if such a system could be developed that was successful in eliminating noise due to short-duration spikes even where the spikes were near the center of k-space. It would additionally be advantageous if such a system was not overly frequency sensitive in its operation, such that it could be easily implemented in a variety of MRI systems having a variety of frequencies of operation, or in MRI systems that operated at multiple frequencies of operation. It would further be advantageous if the system was low in cost and could be easily implemented.
The present invention relates to, in a magnetic resonance imaging system, a method of processing a magnetic resonance signal including transient spike noise. The method includes receiving an initial signal related to the magnetic resonance signal, the initial signal including a carrier signal modulated by a modulation signal and further including a transient spike noise component. The method further includes determining an envelope signal indicative of an envelope of the initial signal, such that the envelope is indicative of the modulation signal and further indicative of the transient spike noise component. The method additionally includes filtering the envelope signal by way of a high-pass filter to remove information relating to the modulation signal from the envelope signal and to produce a filtered envelope signal indicative of the transient spike noise component. The method further includes comparing a comparison signal related to the filtered envelope signal with a threshold to produce a noise indication signal, and modifying the magnetic resonance signal based upon the noise indication signal.
The present invention also relates to a method of processing a magnetic resonance signal including transient spike noise, in a magnetic resonance imaging system. The method includes receiving an initial signal related to the magnetic resonance signal, where the initial signal includes a carrier signal modulated by a modulation signal and further includes a transient spike noise component. The method additionally includes determining an envelope signal indicative of an envelope of the initial signal, such that the envelope is indicative of the modulation signal and further indicative of the transient spike noise component. The method also includes filtering the envelope signal by way of a filter to remove information relating to the modulation signal from the envelope signal and to produce a filtered envelope signal indicative of the transient spike noise component. The method further includes processing the magnetic resonance signal based upon the filtered envelope signal.
The present invention additionally relates to, in a magnetic resonance imaging system, a system for processing a magnetic resonance signal having a transient spike noise component. The system includes an envelope detector that receives an initial signal related to a magnetic resonance signal and provides an envelope signal in response to the initial signal. The system further includes a high-pass filter that is coupled to the envelope detector, receives the envelope signal and provides a filtered envelope signal in response to the envelope signal. The high-pass filter isolates signal components corresponding to transient spike noise in the initial signal from signal components corresponding to magnetic resonance imaging information in the initial signal. The system additionally includes a comparison device that is coupled to the high-pass filter, receives a comparison signal related to the filtered envelope signal and provides a noise indication signal based upon the filtered envelope signal. The system further includes a modification device that is coupled to the comparison device and modifies the magnetic resonance signal based upon the noise indication signal, in order to reduce the transient spike noise component in the magnetic resonance signal and produce an improved magnetic resonance signal.
The present invention additionally relates to a magnetic resonance imaging system. The system includes an operator console, a computer system coupled to the operator console and including a memory, a system control coupled to the computer system and including a transceiver, and a magnet assembly coupled to the system control that produces varying magnetic fields as determined by the system control. The transceiver receives a magnetic resonance signal from the magnet assembly, detects an envelope of an initial signal related to the magnetic resonance signal to produce an envelope signal, and filters the envelope signal to isolate a transient spike noise component of the envelope signal and produce a filtered envelope signal indicative of the transient spike noise component. The transceiver further processes the magnetic resonance signal based upon the filtered envelope signal to eliminate transient spike noise from the magnetic resonance signal and to produce an improved magnetic resonance signal.